Operational amplifiers have many different applications in analog circuit and system design. For instance, operational amplifiers are often used in analog-to-digital (A/D) converters. An ideal operational amplifier has infinite gain, infinite input resistance, and zero output resistance. An actual operational amplifier is typically designed to closely approximate the characteristics of the ideal operational amplifier.
The trend in integrated circuit (IC) design is towards reduced geometries and power supply voltages. However, transistors used in deep sub-micron CMOS processes typically have shorter channel lengths. Transistors having shorter channel lengths tend to have lower output impedance. Furthermore, the use of lower power supply voltages makes it impracticable to stack as many devices between power supplies. As a result, it is becoming increasingly difficult to use standard analog techniques such as single-stage operational amplifiers to achieve high gains typically required by A/D converters and other analog signal processing circuits.